Dispensers have been developed which consist of an inverted container for receiving the liquid and having a bottom outlet positioned inside a reservoir. A pool of the liquid accumulates under atmospheric pressure in the reservoir around the bottom outlet to thereby trap the liquid in the container with a negative pressure above the liquid. Various structures have been provided to disturb the balance of the liquid and the negative pressure in the container to thereby cause controlled flow from the dispenser through an outlet associated with the reservoir and then, after dispensing to cause suck-back to clear liquid from the dispensing outlet. Such dispensers can be seen in applicant's prior patents, namely U.S. Pat. Nos. 4,324,349, 4,635,828, 4,645,097, 5,033,653, and 5,217,147.
The principles of operation of the dispensers are described in some detail in U.S. Pat. No. 5,033,653 which includes diagrams illustrating the conflicting parameters associated with providing temperature compensation. As demonstrated in that patent, the liquid must not flow from the dispenser when temperature rises, and yet the dispenser must have a response rate which will allow the user to dispense almost instantaneously upon applying a force to the dispenser. U.S. Pat. No. 5,033,653 also provides structures which overcome these disadvantages satisfactorily by separating the two parameters so that response rate will not be so dependent on the amount of temperature compensation provided in the structure. However it has been found that when liquids of higher viscosities are used, there is a disadvantage in all of the prior structures which was not apparent earlier. It has now been found that when the dispenser recovers from dispensing by receiving air from outside, instead of liquid returning into the container from the reservoir, air will tunnel directly through the viscous liquid and find its way into the container. This results in excessive build up of liquid in the reservoir and eventual dripping and improper dispensing.
A second cause of the same difficulty results from the rate of recovery of the dispenser after squeezing the container to dispense. The initial force driving the recovery rate is greatest when the container is at its maximum deflection where the most energy is available to drive the recovery. As a result, air will initially be inspired into the dispenser at a high rate and this will cause the air to tunnel through the liquid and into the container.
These disadvantages are exacerbated in smaller structures such as those used to contain hand soap or shampoo. Inherently with these structures the distances and dimensions are such that both of these disadvantages take place at the same time.
It has now been found that different approaches to the relationship between temperature compensation and response rate have to be considered if higher viscosity liquids are to be dispensed, particularly from small dispensers.